A number of heat generative resistances (heat elements) are disposed on a thermal-head substrate at a high density (6 to 10 dots/mm). The pattern electrically connecting such resistances must be formed by the thin film method. The thin film method is as follows. First, a conductive film is coated over a substrate by the method of vaporization or sputter of a metal. Then, a resist agent is coated over this metallic film. The resist is exposed through a mask having openings corresponding to the pattern to be made. The resist is then developed and forms the desired shape of pattern on the metallic film. After that, the metallic film is etched so that the desired pattern of the conductive film is formed in the substrate. Such a thin film method is suitable for forming minute patterns. However, it involves many processing steps and requires much time, labor, and expense.
Another method for forming a conductive pattern on the substrate is the thick film method. In the thick film method, a conductive paste including pulverulent metal is printed on the substrate in the form of the desired pattern through a screen of the pattern to be made. The printed paste is heated or cured so that the desired conductive pattern is obtained. Compared with the thin film method, the thick film method features fewer processing steps and requires less time and labor since the resist coating, exposure, development, and etching processes are unnecessary. However, the thick film method suffers from problems concerning pinholes, curing temperature, resistance of the produced conductive pattern, and density of the pattern.
The crossover portion of the thermal-head comprises lower conductors which connect to the heat generative resistances and upper conductors which are disposed over the lower conductor through an insulating layer. In the conventional method of forming the crossover, the upper conductors are formed by the thin film method, due to reasons described later. Therefore, much time and labor have been required, which has caused low productivity and high costs.
Another conventional method of forming the crossover is to prepare a printed board or a chip, on which the crossover portion is formed, separately from the thermal-head substrate. This allows any convenient heat generative process to be utilized to form the crossover since the conductive pattern on the substrate is not affected by the heat during the crossover manufacturing process. However, such a separate printed board or chip must be connected to the lower conductor of the substrate by the wire bonding method. Such wire bonding between minute patterns is very troublesome and reliability is low.